1. Field of the Invention
The present invention relates generally to the field of oilfield exploration, production, and testing, and more specifically to additives for oil-based drilling fluids for filtration control, suspension, lubrication and lost circulation, and their uses in such applications.
2. Description of Relevant Art
A drilling fluid or mud is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The various functions of a drilling fluid include removing drill cuttings from the wellbore, cooling and lubricating the drill bit, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts.
An important property of the drilling fluid is its rheology, and specific rheological parameters are intended for drilling and circulating the fluid through the well bore. The fluid should be sufficiently viscous to suspend barite and drilled cuttings and to carry the cuttings to the well surface. However, the fluid should not be so viscous as to interfere with the drilling operation.
Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation. Oil based muds are normally used to drill swelling or sloughing shales, salt, gypsum, anhydrite and other evaporate formations, hydrogen sulfide-containing formations, and hot (greater than about 300 degrees Fahrenheit (“° F.”) holes, but may be used in other holes penetrating a subterranean formation as well.
An oil invert emulsion-based drilling fluid may commonly comprise between about 50:50 to about 95:5 by volume oil phase to water phase. Such oil-based muds used in drilling typically comprise: a base oil comprising the external phase of an invert emulsion; a saline, aqueous solution (typically a solution comprising about 30% calcium chloride) comprising the internal phase of the invert emulsion; emulsifiers at the interface of the internal and external phases; and other agents or additives for suspension, weight or density, oil-wetting, fluid loss or filtration control, and rheology control. Invert emulsion-based muds or drilling fluids (also called invert drilling muds or invert muds or fluids) comprise a key segment of the drilling fluids industry.
When drilling wellbores in hydrocarbon-bearing formations to recover hydrocarbons worldwide, there is a continuing and growing desire to enhance efficiencies. Minimizing the number of different additives needed for a drilling fluid and minimizing the variation in such additives from well to well, field to field, country to country, is helpful in realizing the efficiency goal. Preventing loss of drilling fluid is also important. Many times, wells are drilled through lost circulation-prone zones prior to reaching a potential producing zone, requiring use of lost circulation materials to reduce losses of drilling fluids in such zones. Typical lost circulation materials for drilling operations, however, have been directed to water-based solutions.
Increasingly, invert emulsion-based drilling fluids are being subjected to ever greater performance and cost demands as well as environmental restrictions. Consequently, there is a continued need and industry-wide interest in new drilling fluids and additives that provide improved performance while still affording environmental and economical acceptance.